Discovery of optical emission from the supernova remnant G108.2-0.6 and its atomic environment

We report the first detection of optical emission from the shell-type Galactic supernova remnant (SNR) G108.2$-$0.6. We obtained H$\alpha$ images and long-slit spectra using the 1.5-m RTT150 telescope to examine the morphological and spectral characteristics of the SNR. We detected several filaments along its north and south regions, which is consistent with its SNR nature. The spectra exhibit [SII]/H$\alpha$ ratios in the range of 0.4$-$1.1, indicating emission from shock-heated gas. The oxygen doublet emission lines [OI]$\lambda$6300, $\lambda$6363 detected in the south region also support the indicator of the presence of shocks. We estimate the electron density using the [SII] 6716/6731 ratio ranging from 15 to 1800 cm$^{-3}$. The spectra show a relatively low shock velocity of $V_{\rm s}$ $\sim$ 80 km s$^{-1}$ with the pre-shock cloud density of $n_{\rm c}$ $\sim$18$-$57 cm$^{-3}$. The H$\alpha$/H$\beta$ ratios show significant variation across the observed regions with extinction $E(B-V)$ ranging from 0.22 to 1.65. We also analyzed the archival HI data and estimated the kinematic distance to the SNR of $\sim$0.8 kpc and dynamical age as $\sim$70$\pm$10 kyr of G108.2$-$0.6.


INTRODUCTION
Supernova remnants (SNRs) provide us with important information on the supernova explosion mechanism, properties of the surrounding medium, and their interaction (see Vink 2020, for a review).Most SNRs have been detected in radio wavelengths due to their nonthermal synchrotron emission (e.g.Dubner & Giacani 2015;Green 2019).Optical observations of SNRs allow us to inspect the SNR properties such as the morphology, the properties of the shocked gas, and the density of the ambient medium (e.g.Stupar, Parker & Filipovic 2008;Sabin et al. 2013;Fesen et al. 2020;Boumis et al. 2022;Domček et al. 2023).The optical emission of SNRs is dominated by line emission, in particular H, and forbidden line emission from oxygen ([O iii], [O i]), nitrogen ([N ii]), and sulfur ([S ii]).The main criterion for optical SNR identification is a line ratio of [S ii]/H ≥ 0.4, which is useful in separating the shock emission of SNRs from photoionized gas (e.g.Fesen, Blair & Kirshner 1985).
G108.2−0.6 is a new faint and large (70 arcmin × 54 arcmin) shell-type radio SNR discovered in the Canadian Galactic Plane Survey (CGPS) at 1420 MHz (Tian, Leahy & Foster 2007).The ★ E-mail:gunaypayli@gmail.com (GP) † corresponding author: hicranbakis@akdeniz.edu.tr(HB) SNR has an elliptical shell-type with a spectral index of  = -0.5 ± 0.1 ( ∼   ).The 1420 and 408 MHz observations show many bright and faint sources around the SNR: bright SNR G109.1−1.0 (southeast), faint SNR G107.5−1.5 (south), the bright H ii regions Sh2−142 (southwest),  and , and the molecular cloud Sh2−152 (southeast).Tian, Leahy & Foster (2007) also presented the multi-wavelength H i, CO, infrared, Xray, and optical investigations of G108.2−0.6.They found that H i emission associated with the SNR in the radial velocity range is from −53 to −58 km s −1 , and the infrared emission is close to the eastern boundary.They also reported that there is no molecular material toward the SNR, and the maps of X-ray (ROSAT) and optical (Palomar Digitized Sky Survey) show no significant emission associated with the SNR.The distance to G108.2−0.6 was estimated to be 3.2±0.6kpc based on the H i observations (Tian, Leahy & Foster 2007).Recently, Zhao et al. (2020) determined the extinction distance of 1.02±0.01kpc from stars in the line of sight.In this work, we continue our search for optical emission from Galactic SNRs (see Bakış et al. 2023), previously discovered in the radio wavelengths, using the 1.5m RTT150 telescope of TÜBİTAK National Observatory (TUG) 1 in Türkiye.We discovered optical filamentary and diffuse emission from G108.2−0.6 through our search.We perform imaging and spectroscopic observations to investigate the optical properties of the SNR and the ambient medium.We also analyze the H i data to investigate the atomic environment of G108.2−0.6.The structure of the paper is as follows.We present observations and data reduction methods in Section 2. Analysis and results are given in Section 3.
Based on these results, we infer the optical properties of the SNR and the ambient medium in Section 4 and give our conclusions in Section 5.

Optical
We performed H imaging of G108.2−0.6 with the 1.5-m RTT150 telescope.The detector is a CCD camera with 2048 × 2048 pixels, each of 13.5 m × 13.5 m, covering 11.1 × 11.1 arcmin2 field of view (FoV).The details of imaging observations are given in Table 1.The imaging data were processed by using standard procedures with Image Reduction Analysis Facility (IRAF) 2 , including bias and flat-field corrections.
Based on the H imaging, the long-slit spectra of G108.2−0.6 were subsequently obtained with the TFOSC (TUG Faint Object Spectrograph and Camera) mounted at the Cassegrain (f/7.7)focus of the RTT150.The grism-15 was used in the spectral range between 3230 and 9120 Å including H, [O iii], H, [N ii], and [S ii] emission lines with the spectral resolution of  ∼ 749.The slit width is 2.38 arcsec, with slit length of 11.1 arcmin and the slit was oriented in the east-west direction.Table 2 lists the log of the spectroscopic observations.The data were reduced using standard procedures with IRAF, including bias subtraction, flat-field correction, wavelength, and flux calibrations.The spectrum of an Iron-Argon lamp obtained during the observations was used for wavelength calibration.For flux calibration, we used spectra taken on the same night of spectrophotometric standard star BD+28D4211 (Oke 1990).

H i
We analyzed the H i line and radio continuum at 1420 MHz taken from a part of the Canadian Galactic Plane Survey (CGPS; Taylor et al. 2003), which was obtained at the Dominion Radio Astrophysical Observatory (DRAO).The angular resolution was 58 ′′ × 80 ′′ for the H i data and 49 ′′ × 68 ′′ for the radio continuum data.The typical noise fluctuation was ∼3 K for the H i data and ∼0.3 mJy beam −1 for the radio continuum data.In Fig. 1, we show the 1420 MHz radio continuum image of G108.2−0.6 taken from the CGPS (Taylor et al. 2003).

Images
We detected optical emission from N (three locations, namely N1, N2, and N3) and S (two locations, namely S1 and S2) regions of G108.2−0.6 using the H filter.We show the position of these regions in Fig. 1 with black boxes.The H and continuum-subtracted H images of the N region are given in Fig. 2. We show the H images of the S region in Fig. 3. Due to bad weather conditions, we could not observe the S region using the H continuum filter.

Spectra
Our long-slit spectra were taken on the bright optical filaments and diffuse emission in the N and S regions of the SNR (their locations are given in Table 2).In Fig. 4, we also show the slit locations on the H images.
The long-slit spectra in the 4800−6800 Å range are presented in Figs 5 and 6, respectively.Line fluxes and their 1 errors were determined using the deblending function in the splot task in IRAF.The rms was measured on either side of the emission feature to compute errors in the measured line fluxes and then averaged.This average rms was then set as  0 in the splot error package, with nerrsample set to 100 and invgain = 0.The line fluxes and line ratios are given in Table 3.

H i analysis and results
According to Tian, Leahy & Foster (2007), the H i features in the velocity range from -58 to -53 km s −1 show suggestive correlations with the radio continuum shell of SNR G108.2−0.6.On the other hand, our analysis of the same H i data found another counterpart of H i which is possibly associated with the SNR.Fig. 7(b) shows the H i integrated intensity map at the velocity range from -8.9 to 0.2 km s −1 , superposed on the radio shell boundary as defined by eye inspection (Fig. 7(a)).We found a cavity-like distribution of H i which shows a nice spatial correspondence with the radio shell boundary.Moreover, we newly found possible evidence for the expanding shell of H i at the same velocity range.Fig. 7(c Table 3. Line fluxes (normalised to (H) = 100) with the 1 errors.The line ratios are also presented.2 for slit locations.The y-axis shows the flux in 10 −14 ergs cm −2 s −1 Å −1 , while the x-axis shows the wavelength in Hβ 4861 [OIII] 4959 [OIII] 5007 Hα 6563 [NII] 6584 [SII] 6716 [SII] 6731 -0.1 0.4 0.9 1.4 1.9 2.4 2.9 3

Optical morphology
Our H survey showed the existence of filamentary and diffuse structures, which are clearly seen and well correlate with the radio morphology as shown in Figs 1−3.The spatial correspondence of the optical emission with the radio is strongest morphological evidence for the emission being due to the SNR.The optical emission from G108.2−0.6 is mostly diffuse (see Figs 1−3).The detection of several filaments is consistent with the SNR emission.As can be seen in the top panel of Fig. 2, several bright, long and parallel H filaments located in the N1 region of the SNR.The bright diffuse emission is seen in the N2 and N3 regions (see medium and bottom panels of Fig. 2).Much shorter and curved filaments are visible in the S1 and S2 regions (see Fig. 3).We note that our S1 region is close to the northeast region of SNR G107.5−1.5 (see Kothes 2003;Bakış et al. 2023).
G108.2−0.6 is a large SNR, and its long and curved filament structure supports the shocks, which are expanding into a large-scale local ambient medium with varying pre-shock densities.
Using the temden routine and assuming a temperature of 10 4 K, we calculated the electron density is ranging from 15 to 1800 cm −3 ) from the [S ii] 6716/6731 ratio (Osterbrock & Ferland 2006).The high  e values (∼1800 and ∼1640 cm −3 ) are found for the N1a and S2c slit locations can be attributed to the interaction of the SNR with a dense medium.
For the N2a, S1a, and S2c slit locations, we used the [O iii] 5007/H ratio and a planar shock model of Hartigan, Raymond & Hartmann (1987) to estimate the shock velocity  s of ∼80 km s −1 .Other slit locations lack [O iii] emission, indicating shock velocities less than about ∼70 km s −1 .
We then derived the pre-shock cloud density ( c ) to be ∼18−57 cm −3 taking the relation from Fesen & Kirshner (1980), where  [S II] is the electron density calculated from the sulfur line ratio.We detected H emission in the N2a, S1a, S2c, and S2d locations and therefore can estimate the extinction in these directions.We found extinction  (−) value of ∼0.22−1.65 using the H/H ratio and assuming that  (−) = 0.664, where  is a logarithmic extinction (see Kaler 1976;Aller 1984).We finally estimated the column density  H of ∼ (1.2−8.9)× 10 21 cm −2 using the relation  H = 5.4 × 10 21 ×  ( −) (Predehl & Schmitt 1995).The extinction and column density values show significant variation across the observed regions.
Overall, our optical spectral investigation shows that G108.2−0.6 is expanding into dense and complex areas, which agrees well with the conclusion reported in Tian, Leahy & Foster (2007).

Atomic environment
Our analysis found alternative counterparts of H i clouds ( LSR : -8.9−0.2 km s −1 ) possibly associated with the SNR.The cavity-like structures of H i shown in Figs 7(b) and 7(c) are likely an expanding H i shell which was generally formed by strong stellar winds from the progenitor and/or supernova shocks (e.g., Koo et al. 1990;Koo & Heiles 1991).When we adopt the systemic velocity of the H i clouds to be ∼3.5 km s −1 , the expanding velocity of the H i cloud gives ∼5 km s −1 , which is roughly consistent with other Galactic/Magellanic SNRs (e.g., Sano et al. 2017Sano et al. , 2019;;Kuriki et al. 2018).The shock front of the supernova is now impacting on the inner wall of the H i expansion shell.

Distance and Age
If the H i cloud at  LSR = -8.9−0.2 km s −1 is physically associated with SNR G108.2−0.6, we may reconsider the distance to the SNR.By adapting the Galactic rotation model with the IAUrecommended values ( 0 = 8.5 kpc, Θ 0 = 220 km s −1 ; Kerr & Lynden-Bell 1986;Brand & Blitz 1993), we obtain a kinematic distance of the H i cloud and SNR G108.2−0.6 as ∼0.8 kpc.This value is roughly consistent with the extinction distance (Zhao et al. 2020).At the distance, we derived the diameter of SNR G108.2−0.6 to be 16.3 pc × 12.6 pc.By assuming the Sedov-Taylor model (Sedov 1959), the dynamical age of SNR G108.2−0.6 can be estimated to be ∼70±10 kyr.

CONCLUSIONS
In this work, we report the first detection of optical emission from G108.2−0.6 based on both H imaging and spectroscopic observations.The filamentary structure of G108.2−0.6 is consistent with its SNR nature.Our spectra exhibit the H4861, [O iii]4959, 5007, [O i]6300, 6363, H6563, [N ii]6584 and [S ii]6716, 6731 lines.The [O i]6300, 6363 lines detected in the S region also support the indicator of the presence of shocks.The [S ii]/H ratios suggest that the shock-excited gas for G108.2−0.6.We found the electron density ranging from 15 to 1800 cm −3 using [S ii] 6716/6731.The spectra show a relatively low shock velocity of  s ∼ 80 km s −1 with the pre-shock cloud density of  c ∼18−57 cm −3 .We also estimated the kinematic distance to the SNR of ∼0.8 kpc and the dynamical age as ∼70±10 kyr using the archival H i data.Further observations with different frequencies are needed to better understand the nature of this SNR.